Several devices already exist for the purpose of detecting the position of a magnetic head in a recording/reading apparatus.
A first prior detector device is described in published French patent application No. 2.458.125, for example, and comprises strain gauges disposed on the head-carrying piezoelectric strip for the purpose of detecting and measuring strip deformation, and thereby determining the position of the head.
This device is relatively complicated to implement given the difficulty of placing strain gauges on the strips.
Further, published French patent application No. 2.512.247 describes a device disposed inside the rotary head-carrying disk and including a coil which is fixed to the disk and which surrounds a ferrite core which is fixed to the head. The coil is connected to a capacitor so as to constitute an oscillator circuit whose frequency of oscillation is determined by the position of the core in the coil, and thus by the position of the head relative to the rotary disk.
This device is relatively bulky and it requires a relatively thick rotary disk, thereby giving rise to various mechanical stresses and difficulties.
Published French patent application No. 2.427.660 describes a device for controlling the position of a piezoelectric element which takes account of the hysteresis effect, and device including an oscillating branch in which said element is itself used as a capacitor.
This prior device is relatively complex given the necessary electronic circuitry.
Proposals have also been made, as is known, for a different type of device (see for example published French patent application No. 2.345.050), in which the deformation and the vibrations of a piezoelectric bimorph strip (carrying the head) are detected by means of the electrical signals delivered by the bimorph strip by virtue of the inverse piezoelectric effect. The top of the strip is constituted by a first conductive layer for controlling strip deformation, and the bottom of the strip is constituted by a second conductive (which is separated from the first by a dielectric layer) for generating of signals representative of deformation.
This prior structure gives rise to a bimorph strip which is complex in structure.
Published French patent application No. 2.344.912 shows a device in which the alignment between the main recording/reading head and the tracks on the tape is corrected by means of two lateral detector heads disposed on either side of the main head on the same piezoelectric strip. The differences in the amplitudes of the signals detected by said lateral heads constitutes an indication of the position of the main head relative to the tracks.
This prior device requires two additional magnetic heads to be provided and this not only increases the price but also gives rise to assembly difficulties and to considerable bulk.
In addition, most of the above-described prior devices share the drawbacks of providing a relative measurement only in that the measurement is performed relative to the element which carries the head. Further, these prior devices require additional electrical connections to be provided between the fixed portion of the drum and the rotating disk, thereby increasing the cost and the bulk of the recording/reading apparatus.
It will readily be understood that there is a desire for a device capable of detecting the axial position of a rotating recording/reading head running past a tape, with the device being simple and compact (compactness is important given that the heads and their control members are located in a small volume where space is at a premium), and capable of performing absolute measurements, i.e. of providing measurements with reference to a fixed element in order to increase the accuracy and the reliability of measurement.
Further, in a different application relating to apparatus for recording and reading video-type disks, as described by U.S. Pat. No. 3,783,196, there exists a device in which the read head has contact means enabling it to track a spiral groove provided in the disk, with variable height bumps disposed at the bottom of the groove constituting data. The device detects the variable distance between the tops of the bumps and the head while the head remains at a constant height. Detection is based on the principle of variation in the capacitance between a first electrode fixed to the head and a second electrode connected to the turntable.
Further, U.S. Pat. No. 4,183,060 described apparatus for recording and reading optically on a video type disk, the apparatus comprising a capacitive probe having its first electrode connected to a lens which is to be maintained at a constant distance from the disk which itself carries the second electrode, with said distance being maintained constant in spite of axial displacements of the surface of the disk as the disk rotates. Two resonant circuits are provided with the sum of the output signals therefrom constituting a position error signal.
These last two prior devices, in addition to being relatively complex because they use oscillating electronic circuits in order to obtain an output signal that varies in frequency, also relate to a field in which the problem to be solved is expressed in terms that are different from those applicable to recording on and reading from a tape.
These prior devices differ in particular in the nature of the data medium, i.e. in the present invention the data medium is one side of a flexible tape which does not lie in a plane, and which has data recorded on tracks which are mechanically indistinguishable, whereas in these prior devices the data is recorded on a plane rigid disk and the data is findable because of the groove.
Further, the respective utilizations are different. The device described in U.S. Pat. No. 4,183,060 provides a "zero" measurement which relates only to tiny variations in head displacement from a fixed reference position, while the device described in U.S. Pat. No. 3,783,196 performs measurements relating to the signal that conveys the data itself, whereas for recording and/or reading tape the important information lies in the absolute position of the head which position may vary over a relatively wide range.
Preferred embodiments of the present invention remedy the above-mentioned prior art drawbacks and provide a detection device which is compact and simple in design.